1. Field of the Invention
The present invention relates to fixed cutter or drag type bits for drilling subterranean formations. More specifically, the present invention relates to drag bits for drilling hard and/or abrasive rock formations, and especially for drilling such formations interbedded with soft and non-abrasive layers.
2. State of the Art
So-called xe2x80x9cimpregnatedxe2x80x9d drag bits are used conventionally for drilling hard and/or abrasive rock formations, such as sandstones. The impregnated drill bits typically employ a cutting face composed of superhard cutting elements, such as natural or synthetic diamond grit, dispersed within a matrix of wear resistant material. As such a bit drills, the matrix and diamonds wear, worn cutting elements are lost and new cutting elements are exposed. These diamond elements may either be natural or synthetic, and may be cast integral with the body of the bit, as in low-pressure infiltration, or may be preformed separately, as in hot isostatic pressure infiltration, and attached to said bit by brazing or furnaced to bit during manufacturing.
Conventional impregnated bits generally exhibit poor hydraulics design by employing a crow""s foot to distribute drilling fluid across the bit face and providing only minimal flow area. Further, conventional impregnated bits do not drill effectively when the bit encounters softer and less abrasive layers of rock, such as shales. When drilling through shale, or other soft formations, with a conventional impregnated drag bit the cutting structure tends to quickly clog or xe2x80x9cball upxe2x80x9d with formation material making the drill bit ineffective. The softer formations can also plug up fluid courses formed in the drill bit causing heat build up and premature wear of the bit. Therefore, when shale type formations are encountered, a more aggressive bit is desired to achieve a higher rate of penetration (ROP). It follows, therefore, that selection of a bit for use in a particular drilling operation becomes more complicated when it is expected that formations of more than one type will be encountered during the operation.
Thus it would be beneficial to design a drill bit which would perform more aggressively in softer less abrasive formations while also providing adequate ROP in harder more abrasive formations without requiring increased WOB during the drilling process.
The present invention comprises a rotary drag bit employing impregnated cutting elements in the form of discrete, post-like, mutually separated cutting structures projecting upwardly from radially extending blades on the bit face, the blades defining fluid passages therebetween extending to junk slots on the bit gage. The cone portion, or central area of the bit face, is of a relatively shallow configuration and is provided with superabrasive cutters in the form of polycrystalline diamond compacts (PDCs) having cutting faces facing generally in the direction of bit rotation. The PDC cutters provide superior performance in interbedded and shaley formations. Bit hydraulics is enhanced by the aforementioned fluid passages, which are provided with drilling fluid by a plurality of nozzles located in ports distributed over the bit face for enhanced volume and apportionment of drilling fluid flow.
In one embodiment, the blades extend generally radially outwardly in a linear fashion from locations within the cone at the centerline of the bit (in the case of blades carrying the PDC cutters in the cone), within the cone but not at the centerline, or at the edge of the cone, to the gage of the bit, where contiguous gage pads extend longitudinally and define junk slots therebetween. In another embodiment, the blades are curved and extend generally radially outwardly in a spiral fashion from the centerline (again, in the case of the blades carrying PDC cutters), within the cone, or at the edge of the cone, to the gage of the bit and contiguous with longitudinally extending gage pads defining junk slots therebetween. The elongated nature of the spiraled blades provides additional length for carrying the discrete cutting structures so as to enhance redundancy thereof at any given radius.